llvm-cov to expose exactly what lines it considers covered

[vmiklos]

Hi,

First, thanks for llvm-cov, it's a great tool -- Rust has 'cargo llvm-cov' that wraps llvm-cov for Rust code and it works nicely, almost.

One pain point is to figure out details when dealing with line coverage. We invoke 'llvm-cov export ... -format=text' to get the JSON output, and that does tell us if line coverage is complete or not (totals -> lines -> percent), and also this data is provided per-file (files -> ... -> summary -> lines -> percent). However, when percent is not 100, it would be useful to show which lines are not covered.

I understand that deciding if a line is covered or not is tricky, but I assume llvm-cov has this info at some stage, since it provides a percentage summary.

So the request would be to expose exactly which lines are not covered. At the moment 'cargo llvm-cov' tries to infer this from the current JSON output, but that's not perfect in all cases. When it works, it allows us to produce output like this:

Uncovered Lines:
src/test.rs: 15, 16

But for this to work, we need the code at https://github.com/taiki-e/cargo-llvm-cov/blob/09262deeaaabb18889ef265debc6c54600bd1d1a/src/json.rs#L208 to produce this, and feels like llvm-cov already has this info at some point, it's just thrown away.

Thanks.